For me, it has always been the megaman battle network series (3 being my favorite story-wise and 6 in terms of game mechanics).
They never fail to bring me back to my childhood, the wonders of the internet’s potential and so much roleplay potential.
How about you?
MM Zero 2
The zero games are difficult for sure. The only ones I finished without cheats were megaman zero 3 and 4.
I liked them all. And I liked the difficulty. The first one is just hard to go back to and replay after the QoL changes from 2 onward.